1. Field of the Invention
The present invention relates to a charging device for use in an image forming apparatus, such as a copying machine, a facsimile machine, a printer, or other similar image forming apparatuses.
2. Background of the Art
In an electrophotographic image forming apparatus, such as a copying machine, a facsimile machine, a printer, etc., a charging device that charges an image carrier such as a photoreceptor, has mainly performed corona electric discharge. FIG. 1 is a schematic view of an image forming unit of a conventional image forming apparatus including a non-contact type charging device. Arranged around a photoconductive drum 51 are a charger 52, a developing device 53, a transfer device 54, a sheet separation device 55, a cleaning unit 56, and a discharging lamp 57. The charger 52 functioning as a non-contact type charging device performs corona electric discharge.
The charging device that performs corona electric discharge has problems, such as, production of a large amount of ozone. Further, the charging device requires a high-voltage power supply that applies a high voltage in a range of 5 kV to 10 kV to perform corona electric discharge, thereby increasing the cost of the image forming apparatus.
For this reason, a contact type charging device in which a charging member is brought into contact with an image carrier has been proposed, recently. The contact type charging device charges an image carrier without performing corona electric discharge. Therefore, the contact type charging device is free of the above-described problems of the charging device that performs corona electric discharge. However, the contact type charging device has problems, such as an occurrence of an abnormal image such as image tailing, and increase of an abrasion amount of an image carrier. Further, when an alternating current (AC) voltage is used as a charging bias application voltage, the contact type charging device typically has a noise problem. Moreover, because toner and paper powders are rubbed against a surface of an image carrier by a charging member of the charging device, the surface of the image carrier and the charging member are stained.
The technology which addresses the above-described problems is described, for example, in published Japanese patent application No. 10-312098. A charging member is prevented from being stained by toner and paper powders while controlling charging bias voltages applied to an auxiliary charging member and a charging member. In a so-called cleaner-less system, an occurrence of an abnormal image, such as, positive ghost is prevented.
As described above, in an electrophotographic image forming process, a charging device that uniformly charges an image carrier such as a photoreceptor, has mainly performed corona electric discharge. When performing the corona electric discharge, products, such as, ozone and nitrogen oxides, are produced. When high density ozone stays in an image forming apparatus, the ozone oxidizes a surface of a photoreceptor, thereby deteriorating the photosensitivity of the photoreceptor and causing the surface of the photoreceptor to be charged insufficiently. As a result, the quality of an image is deteriorated. Further, the ozone accelerates the deterioration of members other than the photoreceptor, thereby reducing the useful life of the members.
It is considered that nitrogen oxides cause image tailing. It is known that nitrogen oxides are produced by electric discharge. When nitrogen oxides react with moisture in the air, nitric acid is produced. Further, when nitrogen oxides react with metal, metallic nitrate salt is produced. In an electric discharging area, ammonium ion is produced as well as nitrogen oxides. The ammonium ion reacts with nitrogen oxides, and produces a chemical compound. These products are high resistant in a low humidity environment. However, when the products are in a high humidity environment, the products react with moisture in the air and become low resistant. Therefore, when a thin film made of nitric acid or nitrate salt is formed on a surface of a photoreceptor, an abnormal image such as image tailing occurs. The reason for the occurrence of such an abnormal image is that, because nitric acid and nitrate salt absorb moisture and become low resistant, an electrostatic latent image formed on a surface of a photoreceptor is deteriorated.
Further, because nitrogen oxides remain without being disintegrated in the air after electric discharge, chemical compounds produced from nitrogen oxides adhere to a surface of a photoreceptor during a period when the surface of the photoreceptor is not charged, that is, during a period when an image forming process pauses. There is a consideration that the chemical compounds penetrate from a surface of a photoreceptor into its inside with time. The substances adhered on a surface of a photoreceptor are removed by scraping the substances off the surface of the photoreceptor in a cleaning process. However, this removing method typically results in the increase of cost and the deterioration of the photoreceptor.
Recently, a contact type charging device has been used. In the contact type charging device, a charging member in contact with or adjacent to a photoreceptor, charges a photoreceptor. For example, a roller-shaped charging member charges a photoreceptor while the charging member is driven to rotate by the rotation of the photoreceptor. As compared to a charging device that performs corona electric discharge, the contact type charging device produces much less ozone. Further, because the voltage applied from the charging device is relatively low, the cost of a power supply decreases, and designing for electric insulation becomes easy. Moreover, problems caused by ozone are reduced.
For example, published Japanese patent application No. 63-7380 describes a contact type charging device in which a roller-shaped charging member in contact with a photoreceptor charges the photoreceptor while the charging member is driven to rotate by the rotation of the photoreceptor. Because nitrogen oxides are also produced in a charging device in which a charging member is in contact with or adjacent to a photoreceptor, an occurrence of an abnormal image such as image tailing cannot be prevented completely. The occurrence ratio of image tailing has been considered to be higher in an adjacent type charging device in which a charging member is adjacent to a photoreceptor than a contact type charging device in which a charging member contacts a photoreceptor, because a voltage applied from the charging member to the photoreceptor to uniformly charge the photoreceptor needs to be high. However, it has been found that the occurrence ratio of image tailing is substantially equal to each other between the adjacent type contact device and the contact type charging device.
Further, in the contact type charging device, a roller-shaped charging member is generally made of a rubber material. If the contact type charging device is stopped to function for a long period of time, the roller-shaped charging member in contact with a photoreceptor may be deformed. In addition, because rubber tends to absorb moisture, electrical resistance of the rubber tends to vary significantly depending on environmental conditions. Moreover, the rubber needs several kinds of plasticizer and active materials to exert its elasticity and to prevent its deterioration. To disperse conductive colorant in the rubber, dispersion coadjuvant may be used. Specifically, because a surface of a photoreceptor is made of amorphous resins, such as, polycarbonate, and acrylic, it is weak against the plasticizer, the active material, and the dispersion coadjuvant. Moreover, in the contact type charging device, foreign substances may be stuck in a position between a charging member and a photoreceptor, thereby staining the charging member, and resulting in a charging failure. Further, because a roller-shaped charging member directly contacts a photoreceptor, if the charging member is stopped to function for a long period of time, the photoreceptor may be contaminated by the charging member. In this case, an image failure, such as, lateral streak images may occur.
Accordingly, a method for addressing the above-described problems, such as, photoreceptor contamination, and deformation of a roller, has been proposed. In the method, a charging roller is disposed in a non-contact relation to a photoreceptor. When applying a direct current (DC) bias voltage to a surface of a photoreceptor by such a non-contact type charging roller, the potential of the charged surface of the photoreceptor depends on a gap distance between the charging roller and the photoreceptor as well as a charging bias application voltage.
To address a problem, such as, an occurrence of an abnormal image due to the change of the potential of a charged surface of a photoreceptor caused by the change of a gap distance, published Japanese patent application No. 7-287433 describes a technique in which electric discharge is stably performed by providing minute concave and convex portions on a discharging surface of a charging member. With such minute concave and convex portions, even if a gap distance between a charging member and a photoreceptor is changed, the photoreceptor is uniformly charged without occurrence of abnormal images.
However, these minute concave and convex portions may become flattened with time under severe discharging conditions. If the minute concave and convex portions become flattened, desirable effects may not be obtained, and a surface of a photoreceptor may not be uniformly charged over a long period of time. Further, because the range of the minute concave and convex portions of a charging member is limited to uniformly charge the photoreceptor, the process for producing the minute concave and convex portions needs to be strictly controlled. As a result, producing costs for the minute concave and convex portions may increase.
There are two types of method of applying a charging bias to a photoreceptor: (1) a DC voltage is applied to a photoreceptor (hereinafter referred to as a “DC voltage charging”), and (2) a voltage in which an AC voltage is superimposed on a DC voltage is applied to a photoreceptor (hereinafter referred to as a “DC and AC voltage charging”). It is known that a photoreceptor tends to suffer greater damage in an AC voltage charging than in a DC voltage charging. As in the case of the corona electric discharge, products are produced on a photoreceptor due to electric discharge by a charging device. A larger amount of products are produced in the “DC and AC voltage charging” than in the “DC voltage charging”. The reason for this is considered that reverse electric discharge (i.e., electric discharge from a photoreceptor to a charging member) occurs between a charging member and a photoreceptor in the “DC and AC voltage charging”. Therefore, the number of electric discharging in the “DC and AC voltage charging” is much more than that in the “DC voltage charging”. An actual utilization of the “DC voltage charging” is typically difficult due to problems, such as, unevenness of the potential of a charged photoreceptor due to the change of a gap distance, and unstable discharge. Therefore, it is considered to be preferable that the “DC and AC voltage charging” be employed for the non-contact type charging device. However, even in the “DC and AC voltage charging”, if a gap distance significantly varies, electric discharge cannot be stably performed, thereby causing abnormal images.
Recently, another method has been proposed in which a charging member is provided opposite to a surface of an image carrier spaced by a minute gap. With this construction, the charging member is prevented from being stained by contacting the surface of the image carrier. Further, the surface of the image carrier is prevented from being deteriorated quickly.
In this construction, if a gap becomes significantly large, streamer discharge typically occurs. In this condition, the surface of the image carrier cannot be uniformly charged, and spot-shaped abnormal images occur on a toner image formed on the image carrier. As a result, image quality is deteriorated. Therefore, streamer discharge is prevented by setting a gap between the surface of the image carrier and the charging member to about 100 μm or less to enhance image quality.
Published Japanese patent application No. 5-150564 describes a technique in which a frequency of an AC voltage superimposed on a DC voltage to be applied to a charging roller is defined with respect to a linear velocity of a photoreceptor. Generally, if spatial frequency of a charging bias voltage is small, uneven density in an image is sensed by the naked eye. The spatial frequency means the number of peaks or valleys of amplitude cycle of an AC voltage applied to a charging roller per a 1 mm width. For example, the spatial frequency in FIG. 2 is 3/mm.
Further, published Japanese patent application No. 11-84825 describes a technique in which an amplitude and a frequency of an AC voltage applied to a photoreceptor when charging the photoreceptor for not forming latent images thereon is set to be lower than those when charging the photoreceptor for forming latent images thereon. Specifically, as illustrated in FIG. 3, the frequency of a charging bias is set to be high at an image portion, and the frequency of a charging bias at a non-image portion is set to be lower than that. The technique described in published Japanese patent application No. 11-84825 is aimed for preventing an occurrence of image tailing. However, when the present inventors carried out experiments while changing frequency, an amount of products causing image tailing did not change as illustrated in FIG. 4.
As described above, in the background techniques, problems, such as, image deterioration caused by image tailing and contamination of a photoreceptor, uneven density in an image, non-uniform charging, unstable electric discharge, and increase of the cost of an apparatus, are not sufficiently solved.
When charging an image carrier, it has been generally considered that a frequency of an AC voltage superimposed on a DC voltage to be applied to a charging member, such as, a charging roller, is preferably high to uniformly charge the image carrier. However, when the frequency is increased, the surface of the image carrier is deteriorated, and a filming phenomenon typically occurs. In the filming phenomenon, a film made of toner and paper powder adheres to a surface of an image carrier. A film portion of the image carrier is not adequately charged, thereby causing abnormal images. In addition, such a filming phenomenon may also occur at a charging member. On the other hand, if a frequency of an AC voltage is low, uneven image density typically occurs in a halftone image and a solid (black) image.
In a filming phenomenon, a surface of an image carrier is deteriorated due to electric discharge and other factors, thereby causing a film made of toner and paper powder to adhere to the surface of the image carrier. When charging an image carrier by a charging roller, it is generally assumed that molecules of the surface of the image carrier are cut by electric discharge and deteriorated. Further, it is assumed that filming is caused by wax added to toner and adhered to the surface of the image carrier. Moreover, it is assumed that filming is caused by the surface of the image carrier damaged by a cleaning blade or a cleaning brush and carrier particles in a developer. However, these assumptions are not confirmed completely by experiment.
Thus, it is desirable to provide a charging device that prevents an occurrence of a filming phenomenon, extends each useful life of an image carrier and a charging member, and provides a high quality image without uneven density, and to provide an image forming process cartridge, and an image forming apparatus including such a charging device.